Finless sleeper ray
Encyclopedia
The finless sleeper ray (Temera hardwickii) is a little-known species of sleeper ray in the family
Narkidae
, and the sole member of its genus
. It is found over the continental shelf
off Southeast Asia
, from the eastern Andaman Sea
to Vietnam
and Borneo
. Typically growing no more than 15 cm (5.9 in) long, it may be the smallest cartilaginous fish. The finless sleeper ray is the only electric ray that lacks dorsal fin
s. It has an oval pectoral fin disc that varies from longer than wide to wider than long, depending on age, and a short, robust tail that terminates in a short, deep caudal fin. The trailing margins of its pelvic fins are sexually dimorphic, being more concave in males.
Like other members of its family, the finless sleeper ray can generate an electric shock
from paired electric organ
s in its disc, for purposes of defense. Reproduction is aplacental viviparous: the embryo
s hatch within the uterus
and are nourished by yolk. A litter size of four has been recorded from a single individual. The International Union for Conservation of Nature (IUCN) has assessed the finless sleeper ray as Vulnerable
. This slow-reproducing species is caught by intensive bottom trawl and possibly other fisheries throughout its range, which likely cause high mortality
regardless of whether it is discarded. Its numbers off Thailand
seems to have declined significantly.
, who placed it in its own unique genus, in an 1831 issue of the scientific journal
Zoological Miscellany. Gray based his account on two syntype
s collected near Penang
in Malaysia by General Thomas Hardwicke
, and presented to the British Museum
.
, near southern border between Thailand
and Myanmar
, through the Straits of Malacca to Singapore
, and northward to Vietnam
. Its distribution off Thailand is uncertain. There is one record of this species off Sarawak
in Borneo
. This bottom-dwelling species inhabits both inshore and offshore waters on the continental shelf
, over fine sediment
. In the 19th century, it was reportedly abundant year-round in the Straits of Malacca. It remains common in some local areas, although it has become rare off Thailand since 1975.
s of roughly equal size. The rims of the spiracles are smooth. There is a curtain of skin between the small, circular nostrils that reaches the mouth. The nostrils are connected to the corners of the mouth by a pair of grooves. The small mouth is gently arched and protrusible. The teeth are flattened, with hexagonal bases, and are arranged into narrow bands. The five pairs of gill slit
s are short. The pelvic fins are only slightly overlapped by the disc. Each is large, wide, and roughly triangular, with trailing margins that are more concave in males than in females.
The tail is stout and much shorter than the disc. Unique amongst sleeper rays, there are no dorsal fin
s. The tail terminates in a fairly large, triangular caudal fin about as long as wide, with rounded corners. The skin entirely lacks dermal denticles. This species is plain light brown in color dorsally, sometimes with darker markings and whitish spots, and pale ventrally, with broad darker margins on the pectoral and pelvic fins. Possibly the smallest cartilaginous fish, the smallest known adult finless sleeper ray measured 8.2 cm (3.2 in) long and weighed 13 g (0.458561507366456 oz). It seldom exceeds 15 cm (5.9 in) in length, though there is a dubious historical record of a specimen 46 cm (18.1 in) long.
for defense against predators. The shock is generated by a pair of kidney-shaped electric organ
s, each about one-third as long as the ray, situated on either side of the head and visible through the skin. The organs consist of numerous fluid-filled pentagonal or hexagonal columns, which essentially act as batteries
connected in parallel. It presumably feeds on small invertebrate
s. The robust structure of its jaws suggest that it may prefer hard-shelled prey.
The finless sleeper ray is aplacental viviparous, in which the embryo
s hatch from the eggs inside the uterus
, and are sustained over the remainder of the gestation period
by a yolk sac
. There is a record of a female 10.5 cm (4.1 in) long that contained four late-term fetus
es each 2.9 cm (1.1 in) long; the young were similar in form and coloration to the adult, but had thicker discs. Males reach sexual maturity
at around 8.2–10.9 cm (3.2–4.3 in) in length, and females at around 10.5–14.8 cm (4.1–5.8 in) in length. This species is often parasitized by minute worms under the skin, identified as "entozoa" (an obsolete grouping that included acanthocephalans
, trematodes
, cestodes
, and nematode
s) and named Cysteocercus temerae by Cantor (1850).
. It is unlikely to survive capture even if discarded. Catches of this species by Myanmar fisheries are known to be sold frequently in Phuket
. Off Thailand, electric ray populations overall appear to be declining. Along with continuing heavy fishing pressure in the region and the finless sleeper ray's low reproductive rate, these factors have led the International Union for Conservation of Nature (IUCN) to assess it as Vulnerable
.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Narkidae
Narkidae
Members of the family Narkidae are commonly known as sleeper rays. They are restricted to the temperate and tropical Indo-West Pacific from South Africa to Japan to Indonesia, and are exclusively marine and are absent from freshwater habitats...
, and the sole member of its genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. It is found over the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
off Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...
, from the eastern Andaman Sea
Andaman Sea
The Andaman Sea or Burma Sea is a body of water to the southeast of the Bay of Bengal, south of Burma, west of Thailand and east of the Andaman Islands, India; it is part of the Indian Ocean....
to Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
and Borneo
Borneo
Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....
. Typically growing no more than 15 cm (5.9 in) long, it may be the smallest cartilaginous fish. The finless sleeper ray is the only electric ray that lacks dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s. It has an oval pectoral fin disc that varies from longer than wide to wider than long, depending on age, and a short, robust tail that terminates in a short, deep caudal fin. The trailing margins of its pelvic fins are sexually dimorphic, being more concave in males.
Like other members of its family, the finless sleeper ray can generate an electric shock
Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
from paired electric organ
Electric organ
In biology, the electric organ is an organ common to all electric fish used for the purposes of creating an electric field. The electric organ is derived from modified nerve or muscle tissue...
s in its disc, for purposes of defense. Reproduction is aplacental viviparous: the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s hatch within the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
and are nourished by yolk. A litter size of four has been recorded from a single individual. The International Union for Conservation of Nature (IUCN) has assessed the finless sleeper ray as Vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
. This slow-reproducing species is caught by intensive bottom trawl and possibly other fisheries throughout its range, which likely cause high mortality
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
regardless of whether it is discarded. Its numbers off Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
seems to have declined significantly.
Taxonomy
The finless sleeper ray was described by British zoologist John Edward GrayJohn Edward Gray
John Edward Gray, FRS was a British zoologist. He was the elder brother of George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray ....
, who placed it in its own unique genus, in an 1831 issue of the scientific journal
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...
Zoological Miscellany. Gray based his account on two syntype
Syntype
In biological nomenclature, a syntype is a term used to indicate a specimen with a special status.In zoological nomenclature, a syntype is defined as "Each specimen of a type series from which neither a holotype nor a lectotype has been designated [Arts. 72.1.2, 73.2, 74]. The syntypes...
s collected near Penang
Penang
Penang is a state in Malaysia and the name of its constituent island, located on the northwest coast of Peninsular Malaysia by the Strait of Malacca. It is bordered by Kedah in the north and east, and Perak in the south. Penang is the second smallest Malaysian state in area after Perlis, and the...
in Malaysia by General Thomas Hardwicke
Thomas Hardwicke
Major-General Thomas Hardwicke was an English soldier and naturalist who was resident in India from 1777 to 1823. After returning to England he collaborated with John Edward Gray in the publication of Illustrations of Indian Zoology .At the age of 22, he joined the East India Company...
, and presented to the British Museum
British Museum
The British Museum is a museum of human history and culture in London. Its collections, which number more than seven million objects, are amongst the largest and most comprehensive in the world and originate from all continents, illustrating and documenting the story of human culture from its...
.
Distribution and habitat
The range of the finless sleeper ray extends from the eastern Andaman SeaAndaman Sea
The Andaman Sea or Burma Sea is a body of water to the southeast of the Bay of Bengal, south of Burma, west of Thailand and east of the Andaman Islands, India; it is part of the Indian Ocean....
, near southern border between Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
and Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....
, through the Straits of Malacca to Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
, and northward to Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
. Its distribution off Thailand is uncertain. There is one record of this species off Sarawak
Sarawak
Sarawak is one of two Malaysian states on the island of Borneo. Known as Bumi Kenyalang , Sarawak is situated on the north-west of the island. It is the largest state in Malaysia followed by Sabah, the second largest state located to the North- East.The administrative capital is Kuching, which...
in Borneo
Borneo
Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....
. This bottom-dwelling species inhabits both inshore and offshore waters on the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
, over fine sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. In the 19th century, it was reportedly abundant year-round in the Straits of Malacca. It remains common in some local areas, although it has become rare off Thailand since 1975.
Description
The pectoral fin disc of the finless sleeper ray is oval in shape; it is slightly wider than long in adults, and circular or longer than wide in juveniles. The eyes are small and bulging, and immediately followed by spiracleSpiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s of roughly equal size. The rims of the spiracles are smooth. There is a curtain of skin between the small, circular nostrils that reaches the mouth. The nostrils are connected to the corners of the mouth by a pair of grooves. The small mouth is gently arched and protrusible. The teeth are flattened, with hexagonal bases, and are arranged into narrow bands. The five pairs of gill slit
Gill slit
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s are short. The pelvic fins are only slightly overlapped by the disc. Each is large, wide, and roughly triangular, with trailing margins that are more concave in males than in females.
The tail is stout and much shorter than the disc. Unique amongst sleeper rays, there are no dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s. The tail terminates in a fairly large, triangular caudal fin about as long as wide, with rounded corners. The skin entirely lacks dermal denticles. This species is plain light brown in color dorsally, sometimes with darker markings and whitish spots, and pale ventrally, with broad darker margins on the pectoral and pelvic fins. Possibly the smallest cartilaginous fish, the smallest known adult finless sleeper ray measured 8.2 cm (3.2 in) long and weighed 13 g (0.458561507366456 oz). It seldom exceeds 15 cm (5.9 in) in length, though there is a dubious historical record of a specimen 46 cm (18.1 in) long.
Biology and ecology
The natural history of the finless sleeper ray is poorly understood. In common with other members of its family, it is capable of generating a moderate electric shockElectric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
for defense against predators. The shock is generated by a pair of kidney-shaped electric organ
Electric organ
In biology, the electric organ is an organ common to all electric fish used for the purposes of creating an electric field. The electric organ is derived from modified nerve or muscle tissue...
s, each about one-third as long as the ray, situated on either side of the head and visible through the skin. The organs consist of numerous fluid-filled pentagonal or hexagonal columns, which essentially act as batteries
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
connected in parallel. It presumably feeds on small invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s. The robust structure of its jaws suggest that it may prefer hard-shelled prey.
The finless sleeper ray is aplacental viviparous, in which the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s hatch from the eggs inside the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
, and are sustained over the remainder of the gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
by a yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
. There is a record of a female 10.5 cm (4.1 in) long that contained four late-term fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
es each 2.9 cm (1.1 in) long; the young were similar in form and coloration to the adult, but had thicker discs. Males reach sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
at around 8.2–10.9 cm (3.2–4.3 in) in length, and females at around 10.5–14.8 cm (4.1–5.8 in) in length. This species is often parasitized by minute worms under the skin, identified as "entozoa" (an obsolete grouping that included acanthocephalans
Acanthocephala
Acanthocephala is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterized by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host...
, trematodes
Trematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes".-Taxonomy and biodiversity:...
, cestodes
Cestoda
This article describes the flatworm. For the medical condition, see Tapeworm infection.Cestoda is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebrates as adults, and often in the bodies...
, and nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s) and named Cysteocercus temerae by Cantor (1850).
Human interactions
The finless sleeper ray is susceptible to intensive, mixed-species fisheries using bottom trawls and perhaps other demersal fishing gear, operating throughout its range and particularly in the Andaman SeaAndaman Sea
The Andaman Sea or Burma Sea is a body of water to the southeast of the Bay of Bengal, south of Burma, west of Thailand and east of the Andaman Islands, India; it is part of the Indian Ocean....
. It is unlikely to survive capture even if discarded. Catches of this species by Myanmar fisheries are known to be sold frequently in Phuket
Phuket Province
Phuket , formerly known as Thalang and, in Western sources, Junk Ceylon , is one of the southern provinces of Thailand...
. Off Thailand, electric ray populations overall appear to be declining. Along with continuing heavy fishing pressure in the region and the finless sleeper ray's low reproductive rate, these factors have led the International Union for Conservation of Nature (IUCN) to assess it as Vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
.